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Subphylum Cephalochordata (Branchiostoma): General Characters, Classifications and Affinities | B.Sc. II Year Zoology

In this article, we will discuss the characters, classifications(Subphylum: Cephalochordata)  and affinities of Branchiostoma.

Characters of Branchiostoma:

General Characters of Branchiostoma:

1. Marine, widely distributed in shallow waters.

2. Mostly sedentary and buried with only anterior body end, projecting above bottom sand.

3. Body small, 5 to 8 cm long, slender, fish-like, segmented and transparent.

4. Body has trunk and tail. Head lacking.

5. Paired appendages lacking. Median fins present.

6. Exoskeleton absent. Epidermis single-layered.

7. Muscles dorso-lateral, segmented into myotomes.

8. Coelom enterocoelous, reduced in the pharyngeal region by development of atrial cavity.

9. Notochord rod-like, persistent, extending from rostrum to tail, hence, the name Cephalochordata.

10. Digestive tract complete. Pharynx large, perforated by numerous persistent gill-slits opening into atrium.

11. Respiration through general body surface. Special organs for respiration absent.

12. Circulatory system well developed, closed without heart and respiratory pigment. Hepatic portal system developed.

13. Excretion by protonephridia with solenocytes.

14. Nerve cord dorsal, tubular without ganglia and brain. Dorsal and ventral nerve roots separate.

15. Sexes separate. About 25 pairs of gonads. Gonoducts lacking. No asexual reproduction.

16. Fertilisation external in sea water.

17. Development indirect, including a free-swimming larva.

18. Cephalochordata has about 30 species of the genus Branchiostoma and all put in the class Leptocardii.

Classifications Of Subphylum: Cephalochordata



Primitive, Degenerate and Specialised Characters of Branchiostoma (Cephalochordata):

Primitive Characters:

Primitive characters are relics from its original ancestors. They are those characters which are developed and perfected in the course of evolution and they generally show progress.

Primitive characters of Branchiostoma are as follows:

1. There is no specialised head.

2. Absence of paired limbs or fins.

3. The epidermis is made of a single row of cells and a true dermis is absent.

4. Coelom enterocoelous.

5. A complete notochord is persistent throughout life with no vertebral column.

6. There is a complete metameric segmentation of myotomes from one end to the other, as in the embryo of vertebrates, and this obvious metamerism is retained throughout life.

7. Alimentary canal is a straight tube without loops, and the midgut diverticulum is a hollow tube. Jaws absent.

8. A peculiar pharynx specialised for ciliary feeding in which sorting of food takes place after a food-laden current of water enters the pharynx, this involves taking a very large volume of water and its continuous removal through numerous gill-clefts.




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